The present invention relates to a method for reducing the acid content of a nitric acid solution by using an electrolysis current capable of reducing HNO.sub.3 at the cathode.
In the field of reprocessing irradiated nuclear fuel and/or breeder materials, and in the field of the removal of radioactive wastes, chemical as well as electrochemical processes are known which are used to reduce nitric acid concentrations in aqueous, radioactive waste solutions. It has been proposed, for example, to denitrate aqueous medium active waste solutions (MAW) with the aid of electrical current. See, M. Heilgeist et al, "Reduktion des mittelaktiven Abfalles durch salzfreie Verfahrensschritte" in, Bericht der Kernforschungszentrum Karlsruhe GmbH No. 2940 (March 1980), entitled "Sammlung der Vortrage anlasslich des 3. Statusberichtes des Projektes Wiederaufarbeitung und Abfallbehandlung am 8.11.1979", in translation, "Reduction of medium active waste by means of saltfree process steps," in Report by KFK GmbH No. 2940, March 1980, entitled "Collection of lectures given on the occassion of the third status report for the reprocessing and waste treatment project, on November 8th, 1979", pages 216 to 234. In this proposal, nitric acid is converted at the cathode of an electrolysis cell, by absorbing electrons, into nitrogen compounds in which the nitrogen has a lower valency. The following table shows the range of possible products.
______________________________________ Current requirement Cathode Valency of (theoretical) Standard-EMK Product X nitrogen [Ah/Mol HNO.sub.3 ] [V] X/HNO.sub.3 ______________________________________ NO.sub.2 /N.sub.2 O.sub.4 +4 26.8 +0.81 HNO.sub.2 +3 53.6 +0.94 NO +2 80.4 +0.96 N.sub.2 O +1 107.2 +1.11 H.sub.2 N.sub.2 O.sub.2 +1 107.2 +0.9 N.sub.2 0 134 +1.24 NH.sub.3 OH.sup.+ -1 160.8 +0.72 N.sub.2 H.sub.5.sup.+ -2 187.6 +0.82 NH.sub.4.sup.+ -3 214.4 +0.88 ______________________________________ X = reduction product StandardEMK = electromotoric Force (emf) Ah = Ampere hours V = Volt (V) X/HNO.sub.3 : means the voltage value of the emf of the halfcell reaction HNO.sub.3 + n .multidot. e = X (the reduction product of HNO.sub.3, see first column of table) combined with the Standard Hydrogen Electrode: 2 H.sup.+ + 2 .multidot. e = H.sub. 2.
Whether the above-listed products result, which of the above-listed products result and to which extent the above-listed products result, depends on a number of factors including the potential of the cathode, the electrode material, the presence of possibly existing catalysts, the electrolyte concentration and the cell structure. Titanium and graphite have been found to be suitable cathode materials, based on their electrochemical suitability for nitric acid reduction as well as because of their low corrosion rates. Electrochemical nitric acid reduction at graphite cathodes leads, selectively, to NO or ammonia or ammonium ions, respectively. For NO production, a relatively low limit of current density was observed, which limited the conversion. For NH.sub.3 as the product, much higher current densities are required.
The drawbacks of these processes are that, on the one hand, nitrous gases are produced which must be removed in further process steps and, on the other hand, NH.sub.3 is formed which is bound in the nitric acid solution to ammonium nitrate NH.sub.4 NO.sub.3 (hereinafter referred to as AN). The AN must also be removed since it interferes with the further processing of the waste solution containing the AN to a solid product fit for permanent storage.